Field of Invention
The present invention relates to a method and system for substrate processing, and more particularly to a method and system for vaporizing a liquid-phase precursor for use in a substrate processing system.
Description of Related Art
During material processing, such as semiconductor device manufacturing for production of integrated circuits (ICs), vapor deposition is a common technique to form thin films, as well as to form conformal thin films over and within complex topography, on a substrate. Vapor deposition processes can include chemical vapor deposition (CVD) and plasma enhanced CVD (PECVD). For example, in semiconductor manufacturing, such vapor deposition processes may be used for gate dielectric film formation in front-end-of-line (FEOL) operations, and low dielectric constant (low-k) or ultra-low-k, porous or non-porous, dielectric film formation and barrier/seed layer formation for metallization in back-end-of-line (BEOL) operations, as well as capacitor dielectric film formation in DRAM production.
In a CVD process, a continuous stream of film precursor vapor is introduced to a process chamber containing a substrate, wherein the composition of the film precursor has the principal atomic or molecular species found in the film to be formed on the substrate. During this continuous process, the precursor vapor is chemisorbed on the surface of the substrate while it thermally decomposes and reacts with or without the presence of an additional gaseous component that assists the reduction of the chemisorbed material, thus, leaving behind the desired film.
In a PECVD process, the CVD process further includes plasma that is utilized to alter or enhance the film deposition mechanism. For instance, plasma excitation can allow film-forming reactions to proceed at temperatures that are significantly lower than those typically required to produce a similar film by thermally excited CVD. In addition, plasma excitation may activate film-forming chemical reactions that are not energetically or kinetically favored in thermal CVD.
Other CVD techniques include hot-filament CVD (otherwise known as hot-wire CVD or pyrolytic CVD). In hot-filament CVD, a film precursor is thermally decomposed by a resistively heated filament, and the resulting fragmented molecules adsorb and react on the surface of the substrate to leave the desired film. Unlike PECVD, hot-filament CVD does not require formation of plasma.
Oftentimes, liquid phase film precursors are used in any one of the techniques described above, and thus, a vaporization system is required to produce film precursor vapor for delivery to the substrate. However, conventional vaporization systems, or evaporators, suffer from various deficiencies. First, current designs of evaporators include large liquid delivery line and chambers that allow low pressure flash vaporization at the control valve. Large hollow spaces in some evaporation chambers require wall temperatures to be elevated beyond desirable levels to overcome the thermal resistance of the low pressure carrier gas. Chemical contact with these surfaces induces decomposition of the liquid-phase precursor. Flash vaporization of the liquid-phase precursor leaves residue that eventually clogs or inhibits proper valve operation. Second, premature heating or over-heating of liquid-phase precursor causes chemical decomposition. Third, selective vaporization of carrying solvent leaves more viscous chemical to clog the delivery line. Water contamination of liquid-phase precursor or other liquids, for example, initiator chemicals, etc. can react with liquid-phase precursor and also clog the delivery line.
Fourth, recirculation zones in most vaporizer designs trap the chemicals, such as the liquid-phase precursor, for extended periods of time. Chemical decomposition generally increases with time, sometimes dramatically, depending on the liquid-phase precursor used. Fifth, nozzles in the vaporizer and their ability to generate small droplets are generally evaluated at atmospheric pressure. In a vacuum, they are not nearly as efficient and generate larger droplets than expected. Droplets of the liquid-phase precursor cool dramatically from evaporation and have to gain heat from surrounding gas to continue vaporizing. Evaporation is difficult in a vacuum environment with few gas molecules. Droplets of the liquid-phase precursor exist for longer periods and reduce in size slower than at atmospheric pressure. Vacuum gas flows often cannot entrain the droplets of the liquid-phase precursor so the droplets of the liquid-phase precursor hit the overheated vaporizer chamber walls and a portion flash evaporates and the rest decomposes. Decomposition of part of the liquid-phase precursor causes low deposition rate and poor deposition uniformity or even more disastrous results. There is a need for a vaporizer and a method that addresses the above operational issues in substrate processing.